X-Ray Scanning System

ABSTRACT

This specification discloses methods and systems for generating a stereo image of an object that is positioned within an imaging volume. The object is positioned within the imaging volume. Two stationary X-ray source points are selected and activated. X-rays from both stationary X-ray source points are transmitted through the object being scanned and detected using detector elements positioned across the imaging volume and opposite the stationary X-ray source points. Image data sets from the X-rays detected by the detector elements are generated and then combined to produce the stereo image.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/304,738, filed on Nov. 28, 2011, which, in turn, is acontinuation of U.S. patent application Ser. No. 12/697,073, filed onJan. 29, 2010, and now U.S. Pat. No. 8,085,897, issued on Dec. 27, 2011,which, in turn, is a continuation of U.S. patent application Ser. No.10/554,570, issued as U.S. Pat. No. 7,684,538, issued on Mar. 23, 2010,which is a National Stage Application of PCT/GB2004/001747, filed onApr. 23, 2004 and which calls priority to Great Britain PatentApplication Number 0309379.6, filed on Apr. 25, 2003, for priority.

All of the above-mentioned priority applications are herein incorporatedby reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to X-ray scanning It has particularapplication in medical computed tomography (CT) scanning, although itcould equally be used in other suitable applications.

BACKGROUND OF THE INVENTION

X-ray computed tomography scanners have been used in medical imaging formany years. A conventional system comprises an X-ray tube that isrotated about an axis with an arcuate X-ray detector array also rotatedat the same speed around the same axis. The patient is placed with theircentre of gravity close to the axis of rotation, and moved along theaxis as the tube is rotated. A fan-beam of X-radiation passes from thesource through the patient to the X-ray detector array.

The X-ray detector array records the intensity of X-rays passed throughthe patient at each location along its length. From these recorded X-rayintensities, it is possible to form a tomographic (cross-sectional)image, typically by means of a filtered back projection algorithm, ifone set of projection data is recorded at each source angle. In order toproduce an accurate tomographic image of an object, such as a part ofthe patient, it can be shown to be a requirement that the X-ray sourcepass through every plane through the object. In the arrangementdescribed above, this is achieved by the rotational scanning of theX-ray source and the longitudinal movement of the patient.

In this type of system the rate at which X-ray tomographic scans can becollected is dependent on the speed of rotation of the gantry that holdsthe X-ray source and detector array. In a modern medical gantry, theentire tube-detector assembly and gantry will complete two revolutionsper second. This allows up to four tomographic scans to be collected persecond.

As the state-of-the-art has developed, the single ring of X-raydetectors has been replaced by multiple rings of X-ray detectors. Thisallows many slices (typically up to 8) to be scanned simultaneously andreconstructed using filtered back projection methods adapted from thesingle scan machines. In a further improvement of this process, thepatient position may be moved along the axis of the scanner such thatthe source describes a helical motion about the patient. This allows amore sophisticated cone beam image reconstruction method to be appliedthat can in principle offer a more accurate volume image reconstruction.The combination of physical motion of the patient and source rotationabout the patient when combined with multiple ring X-ray detectorsallows volume images of the patient to be obtained over a period ofseveral seconds.

SUMMARY OF THE INVENTION

Swept electron beam scanners have been demonstrated whereby themechanical scanning motion of the X-ray source and X-ray detectors iseliminated, being replaced by a continuous ring (or rings) of X-raydetectors that surrounds the patient with a moving X-ray source beinggenerated as a result of sweeping an electron beam around an arcuateanode. This allows images to be obtained more rapidly than inconventional scanners. By simultaneous movement of the patient along theaxis of the scanner, volume image data may be acquired in timescales ofthe order of a second.

The present invention provides an X-ray imaging system comprising amulti-focus X-ray source extending around an imaging volume to be imagedby the system, and defining a locus of source points from which X-rayscan be directed through the imaging volume, and an X-ray detector arrayalso extending around the imaging volume and arranged to detect X-raysfrom the source points which have passed through the imaging volume,wherein the source points are arranged to follow a three-dimensionallocus around the imaging volume such that data from the detector arraycan be used to produce a three dimensional tomographic image of astationary object within the imaging volume.

Preferably the detector array is substantially cylindrical and saidlocus covers at least half of the circumference of the cylinder, morepreferably the full circumference, and substantially the whole of thelength of the cylinder. More preferably the locus is substantiallyhelical.

However, it will be appreciated that other locus configurations couldequally be used which would enable the object in the imaging volume tobe fully tomographically imaged. Preferably the locus passes throughsubstantially every plane which passes through the imaging volume.

The system preferably further comprises control means arranged to scanthe imaging volume by activating each of the X-ray source points andcollecting respective image data sets, and imaging means arranged toproduce a three-dimensional image of the imaging volume from the datasets. Preferably the control means is arranged to scan the imagingvolume repeatedly to produce consecutive images of the imaged volume.Still more preferably the system further comprises display meansarranged to display the consecutive images to produce a real-time videoimage of the imaged volume.

Preferably the control means is further arranged to activate one of thesource points to produce a plane image of an object and to store theplane image for display. More preferably the control means is arrangedto activate said one of the source points repeatedly thereby to producea series of plane images, and to display the plane images in sequence toproduce a plane video image. Still more preferably the control means isarranged to alternate between a first mode in which it produces a planeimage data set and a second mode in which it produces a tomographicimage data set, and to process the data sets to produce a combined imagedata set for producing a combined display.

The plane image may comprise a fluoroscopic image. Such plane images,especially when used to generate a real time video image, are used for avariety of purposes, including the monitoring of medical operationswhere the position of instruments such as catheters inside a patient canbe monitored in real time.

Indeed, the present invention further provides an X-ray imaging systemcomprising an X-ray source defining plurality of source points around animaging volume from which X-rays can be directed through the imagingvolume, and an X-ray detector array extending around the imaging volumeand arranged to detect X-rays from the source points which have passedthrough the imaging volume, and control means arranged to alternatebetween a first mode in which it controls the source to produce X-raysfrom one of the source points to produce a plane image data set and asecond mode in which it controls the source to produce X-rays from eachof the source points to produce a tomographic image data set, and toprocess the data sets to produce a combined image data set for producinga combined display.

Rather than producing just one plane image, a plurality of source pointscan be used to produce a plurality of plane images in different planes.The control means may arranged to activate a further one of sourcepoints close to said one of the source points whereby a pair of datasets are produced, and to combine the data sets so that the plane imageor each of the plane images is a stereo image. Preferably the controlmeans is arranged to process the data sets by mapping features from oneof the data sets onto the other of the data sets thereby to enhance theimage produced from said other of the data sets.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described byway of example only with reference to the accompanying drawings inwhich:

FIG. 1 is a schematic perspective view of an X-ray scanner according toa first embodiment of the invention;

FIG. 2 is a cross section through the scanner of FIG. 1;

FIG. 3 is a system diagram of a scanner system including the scanner ofFIG. 1;

FIG. 4 is a schematic perspective view of the scanner of FIG. 1reconfigured according to a second embodiment of the invention;

FIG. 5 is a cross section through the scanner of FIG. 1 reconfiguredaccording to a third embodiment of the invention;

FIG. 6 is a schematic perspective view of an X-ray scanner according toa second embodiment of the invention;

FIG. 7 is a schematic perspective view of an X-ray scanner according toa third embodiment of the invention; and

FIG. 8 is a schematic perspective view of an X-ray scanner according toa fourth embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, an X-ray scanner 10 comprises a cylindricalmulti-element detector array 12 formed from many hundred individualrings 14 of detector elements 16. Each ring 14 may typically be of width1-3 mm with centre-to-centre spacing between individual detectorelements in the ring of 1-3 mm. The diameter of the detector array 12 istypically in the range 60-80 cm. The individual detector elements 16should preferably have good efficiency at detecting X-rays and can bemanufactured, for example, from high density scintillators,semiconductor materials or pressurized gas ionization chambers. Thedetector array 12 has a longitudinal central axis Z, and is arranged toenable a patient 18 to be placed inside the array 12 approximately onthe central axis Z.

A multi focus X-ray source 20 is wrapped around the outside of the X-raysensor array 12 in a helical manner as shown in FIG. 2. The source 20allows X-rays to be emitted from each of a number of source points 22spaced along the source 20. X-rays from the multi focus X-ray source 20pass through a clear helical slot 24 that is present in the detectorarray 12 and aligned with the source points 22 such that, for eachsource point 22, the X-rays irradiate a group of the X-ray detectorelements 16 on the opposite side of the detector array 12.

The slot 24 in the detector array 12 is cut in a way that leads to thelocus 23 of source points 22 as shown in FIG. 1. This helical slot 24,and the resulting helical source trajectory, means that the set of datacollected following X-ray transmission through the patient 26 ismathematically sufficient to form a true three dimensional imagereconstruction. This is because the locus 23 of source points 22 passesthrough every plane passing through the scanning volume 28 which isessentially defined as the volume within the sensor array 12, i.e.radially inside the array 12 and between its two longitudinal ends 30,32.

The multi-focus X-ray source 20 comprises a continuous anode held at ahigh positive potential with respect to a plurality of grid controlledelectron emitters. Each emitter is “turned on” in turn and thecorresponding electron beam irradiates the target, so producingX-radiation from a respective source point 22. By changing the activegrid controlled electron emitter, the effect of moving the X-ray sourcearound the patient can be obtained. The X-ray source 20 is housed in athick housing to avoid irradiating X-ray detectors 16 and othercomponents in the system close to the X-ray source 20. An example of asuitable source is described in our co-pending UK patent application No.0309383.8 X-Ray Tube Electron Sources.

Collimation of the X-rays from the source 20 is important to minimizeradiation dose to the patient 26. The source 20 therefore includescollimators arranged to restrict X-ray beams to only that part of thepatient 26 that lies directly between the source and correspondingdetectors. Some suitable collimation systems are disclosed in ourco-pending UK patent application No. 0309374.7 entitled X-Ray Sources,and also in UK patent application No. 0216891.2 entitled RadiationCollimation.

To form an image of the patient 26, the patient is placed in positionwith the part of their body to imaged within the scanning volume 28.Then, with the patient 26 being kept stationary, each of the X-raysource points 22 is operated in turn to scan the patient, and for eachsource point 22 data from the group of detector elements 16 opposite thesource point 22 is used to form an image frame. All of the image framesproduced in one scan are then processed to form a three-dimensionaltomographic X-ray image of the patient as will be described in moredetail below.

Referring to FIG. 3, the complete X-ray system comprises the multi-focusX-ray tube 22 and detector array 12, which is made up of a number ofsensor blocks 34. Each sensor block comprises an array of detectingelements 16, typically 8.times.4 or 16.times.8 pixels, that areelectronically coupled to suitable amplifiers, sample-and-holdamplifiers, analogue multiplexor and analogue-to-digital converter. Eachsensor block 34 is connected to a respective data acquisition circuit(DAQ) 36 that provides gain and offset correction and, whereappropriate, linearization for input to the image reconstructionprocess. To cope with the high data rates generated by the detectorarray 12, multiple hardwired image reconstruction circuits 38 are usedto process data in parallel from the DAQ circuits 36. The imagereconstruction circuits are connected via a summing circuit 40 tovisualization circuit 42, which in turn is connected to a display 44. Asystem controller 46 is connected to, and controls operation of, theX-ray tube 20 and the detector bocks 34 and other circuits 36, 38, 40,42 and display 44. A user interface 48, which can include, for example,a keyboard, a hand held controller, and action specific control buttons,is connected to the controller 46 to allow a user to control operationof the system.

During each scan the X-ray tube 20 is controlled so that each of thesource points 22 produces a beam of X-rays in turn. The order ofactivation of the source points 22 can be sequential, or can be orderedso as to reduce the thermal load on the tube anode, as described in ourco-pending UK patent application No. 0309387.9 entitled X-ray ScanningFor each scan, data from each of the detector blocks 34 is processed inthe respective DAQ 36 and image reconstruction circuit 38. Thereconstructed images from each reconstruction circuit 38 are summed andpassed to a visualization unit 42 that creates a 3D tomographic image.The images from subsequent scans are combined to form a real time 3Dvideo image which is shown in the display 44.

For equivalent image quality, the faster the scan time, the higher theX-ray tube current. For example, a 5 ms scan time requires an anodecurrent in excess of 500 mA for high quality medical diagnostic imaging.

It will be appreciated that the combination of a helical trajectorymulti-focus X-tray tube 20 and multi-ring X-ray detector 12 with helicalslot 24 allows true full volume tomographic image data to be collectedwith no mechanical movement of X-ray source, X-ray detector or patient.Since no mechanical movement is involved, it is possible to generatevolume images very quickly, with the only limitation being the outputpower of the X-ray tube. The scanner described can therefore providefull three-dimensional X-ray tomographic scans using accurate cone-beamthree dimensional reconstruction algorithms over millisecond timescales.

Applications for the scanner in this mode of operation include volumecardiac imaging (single cycle) where movies of cardiac motion can begenerated over a single cycle. Assume a cardiac cycle time of 800 ms anda 4 ms tomographic scan time, a single cardiac cycle movie will contain200 volume tomographic images. A preferred use of this scanner is incardiac angiography in which iodine contrast agent is passed through theheart and surrounding vessels.

Referring to FIG. 4, in a second mode of operation, the scanner systemof FIGS. 1 to 3 is set up for use in fluoroscopy. This can be singleplane, bi-plane or multi-plane fluoroscopy. For single plane fluoroscopya single source point 22 a is used, and a beam of X-rays passed fromthat source point 22 a, through the patient, and onto a group 17 of thedetector elements 16. The data from the detector elements 16 is used toform an image frame data set which represents a 2 dimensional X-rayprojection image of the imaged volume. This process is repeated insuccessive imaging periods, which may be of the order of 5 ms. It willbe appreciated that this is significantly faster than conventionalfluoroscopy for which the corresponding period is of the order of 40 msor more. In this case the image frame data sets are output directly fromthe DAQs 36 to a frame store 50 from which they can be displayed in turnas images on the display 44 to provide a real time 2D video image of thepatient.

Since a large number of X-ray source points 22 are present in thesystem, it can easily be controlled to alternate between two, three ormore source points 22 b, 22 c spaced around the patient. For each sourcepoint 22 a 22 b, 22 c, a corresponding group of detector elements 16will be used to produce a respective series of fluoroscopic imageframes. By cycling between the source points 22 a, 22 b, 22 csimultaneous video images in a number of planes can be produced. Thesefluoroscopic images can either simply be displayed simultaneously on thedisplay 44 or processed to provide a single video image combiningfeatures from each of the plane video images. The angle between planesmay be adjusted electronically by switching the location of the emittingelectron source. Applications for the system used in this mode areneuroradiology and neuroangiography.

The fluoroscopic images produced can be improved by using the methodsdescribed in UK patent application No. 0216893.8 entitled ImageColouring and UK patent application No. 0216889.6 entitled ImageControl.

Referring to FIG. 5, in a further mode of operation, the system is setup to provide stereo imaging of the imaging volume 28. In this set-up,two source points 22 d, 22 e are used which are close together. Each ofthem is activated in turn to produce a respective transmission imagedata set from a corresponding group of detector elements 16 on theopposite side of the imaging volume 28. These image data sets are storedin the frame store 50. A pair of image frame data sets, one from eachsource point 22 d, 22 e, is combined to produce a stereo image data setrepresenting an image of the imaged volume, and successive stereo imagescan be displayed to produce a real time stereo view video image of theimaged volume 28. The angle between the two sources 22 d, 22 e, andhence the degree of parallax, can be adjusted dynamically to suit thesize of the patient or organ being imaged.

Because the source points 22 to be used, and the order in which they areused, can be controlled by the controller 46 in any suitable order orcombination, it is also possible for the scanner to switch rapidlybetween any of the three modes of operation described above. This willreduce the rate at which data can be collected for each mode, butenables the images produced in each mode to be combined. For example inone mode the scanner is arranged to scan the object repeatedly toproduce a 3D tomographic image of the object, but, between eachsuccessive pair of scans, to use one of the source points 22 to producea 2D flouroscopic image of the object. The tomographic image is thenanalyzed by the visualizing unit 42 to identify specific features, whichare then identified with corresponding features on the fluoroscopicimage. The fluoroscopic image is then enhanced by mapping features fromthe 3D image onto the 2D image using software pointers to show themapped features more clearly. This can be advantageous, for examplewhere one or more features is obscured in the 2D image, or where two ormore features cannot be distinguished from each other. Alternatively,features identified in the fluoroscopic image can be mapped directlyonto the three-dimensional tomographic image. It will be appreciatedthat the automatic registration of the fluoroscopic image and volumetomographic data can be of major clinical advantage.

Similar combinations can be made of the stereo view imaging data and thetomographic imaging data, or indeed of all three imaging methods. Thecombination of volume real-time tomographic imaging, real-timemulti-plane fluoroscopy and real-time stereo view imaging in onespatially registered imaging system can lead to shortening of clinicalprocedures, enhanced diagnosis and, in some cases, a lowering of patientdose.

It will be appreciated that the exact shape of the X-ray source can bemodified substantially. The embodiment described above is the simplestto use in many circumstances as the regular helix with a single turnproduces data which is simple to analyze. However, other shapes ofsource could be used. For example, referring to FIG. 6, in a secondembodiment of the invention, a helical locus 60 of X-ray source points62 is again used, but in this case the helix has a plurality of turnsaround the detector array 64. Referring to FIG. 7, in a fourthembodiment, the locus 66 of source points 68 is not in a helix, but ismade up of two stepped loci 70, 71 each extending half way round thecircumference of the cylindrical detector array 72 and along its fulllength. Finally, referring to FIG. 8, in a fourth embodiment thedetector array 74 is not straight cylindrical, but instead is partspherical being of larger circumference at its centre line 76 than atits longitudinal ends 78, 79. The locus 80 of source points 81 extendsfrom one end 78 of the detector array 74 to the other 79 while followinga single turn around its circumference.

I claim:
 1. A method of generating a stereo image of an object that ispositioned within an imaging volume comprising: positioning the objectwithin the imaging volume; Selecting a first stationary X-ray sourcepoint; selecting a second stationary X-ray source point having apredetermined angle from the first stationary X-ray source point,wherein said angle is determined based upon a size of the object beingscanned; activating the first stationary X-ray source point while theobject remains stationary; detecting X-rays transmitted through theobject being scanned using a first detector element positioned acrossthe imaging volume and opposite the first stationary X-ray source point;activating the second stationary X-ray source point while the objectremains stationary; detecting X-rays transmitted through the objectbeing scanned using a second detector element positioned across theimaging volume and opposite the second stationary X-ray source point;generating a first image data set from the X-rays detected by the firstdetector element; generating a second image data set from the X-raysdetected by the second detector element; and, combining the first imagedata set and second image data set to produce the stereo image.
 2. Themethod of claim 1 wherein X-rays from each of said first and secondX-ray source points is generated by a single stationary X-ray sourcecomprising an anode.
 3. The method of claim 1 wherein the first detectorelement and second detector element are part of a detector array,wherein said detector array comprises a plurality of rings of detectorelements and wherein each of said plurality of rings encompasses aportion of said imaging volume.
 4. The method of claim 3 wherein each ofsaid plurality of rings has a length parallel to a central axis of theimaging volume, wherein the central axis has a length, and wherein thelength of the central axis is greater than the length of each one ofsaid plurality of rings.
 5. The method of claim 1 wherein the firstdetector element is part of a detector array and wherein the first X-raysource point is positioned behind a portion of said detector array. 6.The method of claim 1 wherein the second detector element is part of adetector array and wherein the second X-ray source point is positionedbehind a portion of said detector array.
 7. The method of claim 1wherein X-rays transmitted from the first stationary X-ray source pointpass through a first collimating slot prior to entering the imagingvolume.
 8. The method of claim 7 wherein X-rays transmitted from thesecond stationary X-ray source point pass through a second collimatingslot prior to entering the imaging volume.
 9. The method of claim 8wherein each of said first collimating slot and second collimating slotare formed in a detector array positioned between the first X-ray sourceand the imaging volume and positioned between the second X-ray sourceand the imaging volume.
 10. The method of claim 8 wherein each of saidfirst collimating slot and second collimating slot are part of the sameslot extending a length of a central axis of the imaging volume.
 11. Themethod of claim 10 wherein the slot extending a length of a central axisof the imaging volume has a helical shape.
 12. The method of claim 10wherein the slot extending a length of a central axis of the imagingvolume comprises a plurality of stepped loci.
 13. The method of claim 10wherein the slot extending a length of a central axis of the imagingvolume forms a three-dimensional locus around the imaging volume. 14.The method of claim 13 wherein said locus passes through substantiallyevery plane of the imaging volume.
 15. The method of claim 1, furthercomprising a controller for scanning the imaging volume by activatingeach of the source points.